Up here in the PRM, if you "wait" to register a purchased vehicle, the RMV adds additional "back taxes". I believe the normal, non-penalized period is less than 2 weeks.
So if it's a private sale, I suggest not having the date written down. Just get the sellers signature on the rear of the title (in the correct spot!!!) and on the bill of sale.
Then, when you go to register it, date both the title and BOS with the same date, like a day or 2 prior...
If the seller insists on a date, have them fill out a separate BOS with the date on it, and keep the non-dated one for "the day".